Nano-sized fluorescent labeling materials have been widely used for biological studies and clinical applications. Conventional down-conversion fluorescent labels require ultraviolet or blue excitation wavelength (Beaurepaire et al. 2004; Wang et al. 2006). These single-photon fluorescent labels emit one lower energy photon after absorbing higher energy UV or visible photon. Their disadvantages include low light penetration depth and possible severe photo-damage to living organisms. Furthermore, many biological samples show auto fluorescence under short wavelength UV radiation, which decreases the sensitivity of detection. The use of fluorescent labels that can be excited in the near infrared (NIR) region was suggested (Sertchook and Avnir 2003). Yi et al. 2004 and van de Rijke et al. 2001, suggested the infrared-to-visible up-conversion nanocrystal. These nanocrystals emit one higher energy photon after absorbing two or more lower-energy photons. Different colors of visible light can be obtained from different up-conversion phosphors when excited by the same IR laser (van de Rijke et al. 2001). In comparison with down-conversion fluorescent materials, up-conversion nanocrystals show very low background light due to their unique fluorescent properties. In addition, photo-damage to biological tissues is minimal because these tissues are usually transparent to NIR light (Suyver et al. 2005). Among the most commonly used up-conversion nanocrystals, Yb/Er or Yb/Tm co-doped NaYF4 nanocrystals have been reported as efficient infrared-to-visible up-conversion material (Kramer et al. 2004). Colloidal Yb/Er and Yb/Tm co-doped NaYF4 nanocrystals have been prepared with strong up-conversion fluorescence seven orders of magnitude higher than that of CdSe—ZnS quantum dots (Heer et al. 2004; Larson et al. 2003).
Some efforts have been made to produce up-conversion NaYF4 nanocrystals with controlled size and shape (Heer et al. 2004; Yi et al. 2004; Zeng et al. 2005). Ethylenediamine tetraacetic acid (EDTA) was used as a chelating agent to control the growth of NaYF4 nanocrystals, but the nanocrystals as prepared tended to precipitate in solution (Yi et al. 2004; Zeng et al. 2005). Colloidal solutions of NaYF4 nanocrystals were prepared. However, these nanocrystals were hydrophobic and could only be dispersed in certain organic solvents such as hexane and dimethyl sulfoxide (DMSO) under ultrasound sonication (Boyer et al. 2006; Heer et al. 2004; Mai et al. 2006). Use of these nanocrystals directly for bio-applications is very limited due to their very small solubility in water and unsuitable surface property.
Accordingly, there is still a need in this field of technology of improved upconversion nanoparticles. In fact, the synthesis of monodisperse and water soluble fluoride nanocrystals with upconversion fluorescence is still very challenging. In particular, there is a need in developing suitable methods for synthesizing up-conversion NaYF4 nanocrystals which are dispersible in water and organic solvents and have some functional chemical groups on their surfaces for conjugation of biomolecules (Larson et al. 2003).